Computational Fluid Dynamics Applied To Investigate Development of Highly Conductive Channels Within the Fracture Geometry

Abstract

Summary A conventional proppant pack may lose up to 99% of its conductivity because of gel damage, fines migration, multiphase flow, and non-Darcy flow. Therefore, the concept of pillar fracturing was developed to generate highly conductive paths for hydrocarbon to flow. This paper describes experimental results and numerical models of a new method to generate stable proppant pillars. The proposed treatment method depends on fingering phenomena observed when a less-viscous fluid that does not carry proppant is injected to displace a more-viscous one that carries proppant. Because the low-viscosity fluid tends to move faster than the high-viscosity fluid, the low-viscosity fluid can penetrate the high-viscosity fluid and create channels inside it. This reshapes and divides the proppant-laden, high-viscosity fluid into isolated patterns that create proppant pillars. Large-scale experiments (slot test, 2-ft height and 16-ft length) were performed to evaluate the development and stability of the created channels by use of different injection rates and different fluid viscosities. In addition, a computational fluid dynamics (CFD) model was constructed to simulate the experiment results and to scale them up into full fracture dimensions. The study focused on the effects of surface-injection rate [1 to 40 (bbl/min)/cluster], pulsing time (5 to 60 seconds), and viscosity ratio (from 2 to 200) between the two injected fluids. Experimental results and numerical modeling showed that with the use of viscous-fingering phenomena, a pillar-propped fracture with conductive and stable channels can be created. Channel shape and size were dependent on injection rate where increasing the injection rate reduces the main channel width with increasing the channel branching. Full piston-displacement behavior was noticed after 60% of the fracture height, when a high-viscosity fluid displaced a low-viscosity fluid, and their viscosity ratio was greater than 5. By reducing the viscosity ratio between the two fluids, the created channel shape converts from cylindrical (where the beginning and end of the channel have the same width) to conical behavior (where the beginning of the channel is wider than the end). This explains why the length of the channel decreases with the viscosity ratio between the two fluids. Distance between pillars was reduced with increasing travel distance from the wellbore, or with reduced pulse-stage volume, time, or rate.